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Remove the blocks from behind the baseboard and reattach the existing molding to the wall.
Measure and cut a new section for the gap. Apply wood glue to its mitered ends and adhesive across the back.
Position the board, allowing the glue and adhesive to adhere to the baseboard and wall. Drive in nails to strengthen the joints.
Baseboard molding can sometimes crack open and become unsightly. In most cases, this is because the joint was not properly glued and secured in place when it was installed.
Drill pilot holes through the top and bottom of the joint, making sure that you drill into both mitered edges of the corner (image 1). Apply some wood glue to the joint, and wipe away any excess (image 2). Use finish nails through the pilot holes to secure the miter in position or, for larger baseboards or moldings, use thin-gauge screws (image 3).